The invention relates to a method of making rolls of the type used in calenders and analogous machines. More particularly, the invention relates to improvements in methods of making rolls of the type wherein an elongated core is surrounded by an elastomeric envelope and the envelope includes at least one stack of thin discs.
German Pat. No. 10 71 036 discloses a method of making calender rolls which includes welding the discs to each other subsequent to placing of the discs around a core. The discs are made of fleece and contain a bonding agent, such as latex, in a partially vulcanized state. The first step involves the placing of discs around the core and the discs are thereupon pressed against each other with a substantial force. The core and the thus obtained envelope (consisting of pressed-together discs) are thereupon heated for an extended period of time in order to complete the vulcanizing of the bonding agent. The heating step involves subjecting the discs and the partially vulcanized bonding agent to temperatures in the range of 100.degree.-120.degree. C. for a period of 5-8 hours. The thus obtained envelope has a hardness in the range of 90-97 Shore A. A role which embodies an envelope of the just outlined character can be used as a means for squeezing liquids from leather, textile materials and the like or as a counterrole in embossing and like machines.
A drawback of the aforedescribed rolls is that they cannot be put to use in supercalenders and/or compact calenders, for example, in calenders which are used as a means for smoothing the exposed surfaces of webs of paper, webs of fleece, magnetic tapes, webs of textile material and the like. In such calenders, the rolls contain elastic envelopes which surround a solid shaft or a hollow cylinder made of steel or grey cast iron. The hardness of the envelope forming part of a roll for use in a supercalender or a compact calender must exceed the aforementioned values.
European Pat. No. 0 131 083 B1 proposes to use primarily chemical pulp in the discs of rolls of calenders or the like. For example, an envelope which forms part of a roll satisfying the European standards concerning elastic calender rolls contains 80 percent cotton and 20 percent wool. However, it is also known to employ envelopes which contain up to 50 percent asbestos fibers or up to 15 percent carbon fibers. The characteristics of the roll depend on the ratio of various fibers in its envelope. For example, the resistance of an envelope to permanent deformation (marking) can be increased by increasing the percentage of animal wool, i.e., such envelope is less likely to undergo permanent deformation as a result of repeated engagement with pleats, folds or other accumulations of material in a running web of paper or the like. On the other hand, an increase in the percentage of animal wool entails a reduction of hysteresis and attendant pronounced heating simultaneously with a reduction of the ability to stand mechanical stresses. This eliminates such rolls from use under elevated stresses, namely at high speeds and at pronounced line loads.
The ability of a roll to stand elevated temperatures is also an important factor when the envelope develops heat due to hysteresis and also because a roll having an elastic envelope is heated due to transfer of heat from a companion roll which cooperates with the roll having an elastic envelope to define therewith a nip for a running web of paper or the like. Reference may be had to U.S. Pat. No. 2,987,802 which discloses a method of mixing pulp in a paper making machine with a duroplastic substance. The paper web which is obtained from such mixture is heated to a temperature of 290.degree.-300.degree. C. for a period of 6-10 minutes prior to cutting of discs from the web.